This invention pertains to the art of arc discharge lamp assemblies and more particularly to low pressure fluorescent lamp assemblies that use a high voltage provided by an electronic ballast. The invention is particularly applicable to limiting the arcing associated with a filament or cathode of the lamp assembly burning out and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other environments and applications.
One particular type of arc discharge lamp is a fluorescent lamp that is generally comprised of a sealed glass tube containing a low pressure gas. Normally the gas is a mixture of an inert gas with another substance, such as mercury. Electrode pairs are disposed at opposite ends of the tube and current is supplied to a filament that bridges each of the electrode pairs. Typically, the electrical current supplied to the fluorescent lamp is relatively low and the electrode pairs defining the cathode have an electron emissive material coated thereon. The electrons supplied by the cathode maintain an arc discharge through the gas, and the anode receives the charged particles to complete the circuit. The low pressure gas that fills the glass tube between the cathode and anode undergoes ionization as a result of the arc. For example, typically, the mercury is excited and radiates primarily at 256 nm, an ultraviolet radiation, which in turn excites a luminescent coating, such as phosphor, provided on the interior surface of the glass tube.
An initial voltage is applied across the tube to start a discharge between the cathode and anode. In prior, well known arrangements, a voltage on the order of 400 volts is typical to start the ionization process. In more recent lamp constructions, higher voltages on the order of 800 to 900 volts are now used and provided by an electronic ballast, so called instant start ballasts.
Presently, glass is used to support the cathode lead wires. The support glass is made of a hard glass such as a boro-silicate composition which contains sodium. When the arc burns the lead wire back to the glass support, the sodium enhances the discharge process. It has thus been deemed desirable to limit or preclude the arcing of the lead wire upon failure of the filament or cathode because of undesirable characteristics that result.